In both cases, you'll start by creating a new inventory item. In this example we're including the word "Discount" in the name to make it easy to remember.
To offer a fixed discount amount, select "Discount - Amount" and then enter the amount into the "Price" field. It should look like this:
To enter a percentage discount, select "Discount - Percent" and then enter the percentage into the "Percent Discount" field. It should look like this:
Once your inventory item is created, just add it as a line item to an estimate/invoice, and the discount will be applied!
Discount amount on an invoice:
Discount percent on an invoice: